The present invention relates to a composite plastic film and a greenhouse built therewith as a light-transmitting covering sheet material. More particularly, the invention relates to a double-layered composite plastic film capable of exhibiting, when used as a covering sheet material of a greenhouse for plant growing, a very favorable effect for growing of plants that the decrease in the inside temperature of the greenhouse is relatively small in a winter season and the increase in the inside temperature of the greenhouse is not unduly large in a summer season without substantially decreasing the amount of insolation to the growing plants as well as a greenhouse built therewith.
As is well known, it is a wide prevalence in the agriculture and horticulture that various kinds of crops including vegetables and floral plants as well as some fruit trees are grown under an artificially controlled climatic conditions in a greenhouse built with a framework and covered with a transparent or translucent plastic film spread on the framework in place of traditional glass panes. In some cases, so-called "tunnel growing" is practiced in which each ridge of the plant-growing field is covered with a tunnel-like shell made from a transparent or translucent plastic film supported by semicircular frames so as to protect the growing plants from direct influences of ambient conditions. A greenhouse should have an effect that, when the outside air temperature is low, for example, in winter, dissipation of the heat from inside to the outside of the greenhouse is prevented so as to moderate the temperature decrease in the greenhouse and, when the outside air temperature is too high, for example, in summer with a large amount of insolation, entering of the sunlight energy is decreased so as to facilitate growing of the plants.
Various thermoplastic resins are conventionally used as the material for the above mentioned transparent or translucent plastic films as a covering material of greenhouses including polyvinyl chloride resins, polyolefin resins, polyacrylate resins and the like in the form of a single film, sheet or plate. A problem in a greenhouse built with these conventional plastic films or sheets is that the effect of temperature-modulation to be obtained in the greenhouse is not always high enough so that the artificially controlled climatic conditions are far from ideal for the growing of the plants. Accordingly, it is sometimes attempted to increase the thickness of the plastic film or to use a plastic film in which a substantial amount of a filler is compounded with an object to improve the temperature-modulating effect. These measures are in fact effective to some extent for the modulation of the changes in the climatic conditions in the greenhouse under changes in the ambient temperature while it is unavoidable that transmission of the light of the wavelength effective for the growth of plants is decreased so much if not to mention the increase in the costs of the plastic films.
Accordingly, it is eagerly desired to develop a transparent or translucent plastic film which can be used as a covering material of a greenhouse to exhibit an excellent effect of temperature modulation that the inside temperature of the greenhouse can be kept relatively high when the outside air temperature is low while increase in the inside temperature of the greenhouse can be suppressed when the outside air temperature is high without unduly decreasing the amount of insulation into the greenhouse.